1. Field of the Invention
The present invention relates to microbeads of a crosslinked porous polymeric material and methods for preparing such microbeads. In one embodiment, this invention is directed to a polymeric microbead of exceptionally high porosity.
2. Description of the Prior Art
Crosslinked, homogeneous, porous polymeric materials are disclosed in U.S. Pat. No. 4,522,953 (Barby et al., issued Jun. 11, 1985). The disclosed polymeric materials are produced by polymerization of water-in-oil emulsions having a relatively high ratio of water to oil, typically on the order of 70% or more. These emulsions are termed "high internal phase emulsions" and are known in the art as "HIPEs". HIPEs comprise an oil continuous phase including a monomer and a crosslinking agent and an aqueous discontinuous phase. Such emulsions are prepared by subjecting the combined oil and water phases to agitation in the presence of an emulsifier. Polymers are produced from the resultant emulsion by heating. The polymers are then washed to remove any unpolymerized monomer/crosslinker.
The disclosed porous polymers have rigid structures containing cavities interconnected by pores in the cavity walls. By choosing appropriate component and process conditions, HIPE polymers with void volumes of 80% or more can be achieved. These materials thus have a very high capacity for absorbing and retaining liquids.
Various modifications of HIPE polymers have been described. For instance, U.S. Pat. No. 4,536,521 (Haq, issued Aug. 20, 1985) discloses that HIPE polymers can be sulfonated to produce a sulfonated polymeric material that exhibits a high capacity for absorption of ionic solutions. Other functionalized HIPE polymers prepared by a similar process have been disclosed in U.S. Pat. No. 4,611,014 (Jomes et al., issued Sep. 9, 1986) and U.S. Pat. No. 4,612,334 (Jones et al., Sep. 16, 1986).
Although the existence of polymerizable HIPEs is known, the preparation of useful HIPE polymers is not without its difficulties. Because the emulsions used to produce these polymers have a high ratio of water to oil, the emulsions tend to be unstable. Selection of the appropriate monomer/crosslinker concentration, emulsifier and emulsifier concentration, temperature, and agitation conditions are all important to forming a stable emulsion. Slight changes in any of these variables can cause the emulsion to "break" or separate into distinct oil and water phases. Furthermore, emulsion components and process conditions that produce a stable emulsion may not always yield HIPE polymers that are useful for their intended purpose.
In addition to these problems, the costs associated with scaling up production of HIPE polymers have prevented commercial development of HIPE polymer-based products. Processes for large-scale production of HIPE polymers are known. For instance, U.S. Pat. No. 5,149,720 (DesMarais et al., issued Sep. 22, 1992) discloses a continuous process for preparing HIPEs that are suitable for polymerization into absorbent polymers. In addition, a method that facilitates such continuous processes by reducing the curing time of monomers in a HIPE is set forth in U.S. Pat. No. 5,252,619 (Brownscombe et al., issued Oct. 12, 1993). Large-scale production of HIPE polymers by such known processes, however, has been hampered by the lack of a cost-efficient means of removing the unpolymerized emulsion components from the polymers.
All prior art processes for making HIPE polymers produce a block of polymeric material the size and shape of the vessel used for polymerization. The problem with producing HIPE polymers in block form is that it is very difficult to wash unpolymerized emulsion components out of a block of low density, highly absorbent material. The attempted solution to this problem has been to grind the blocks into particles, but this approach is unsatisfactory because both the drying and milling processes are costly, and there is a limit to the size of the particles produced by milling. For many applications, the removal of residual emulsion components is essential. Yet, to date, no cost-efficient method for performing this wash step has been developed.
An additional problem with prior art HIPE polymeric blocks is that the blocks have a skin that forms at the interface between the HIPE and the container used for polymerization. (U.S. Pat. No. 4,522,953, Barby et al., issued Jun. 11, 1985, at column 4, lines 1-6). To produce a permeable block, and hence, to produce a useful product, the skin must be removed. Ideally, one would like to be able to produce a polymeric material having the desirable characteristics of HIPE polymers but lacking the skin.